One important aspect of teacher effectiveness, of course, is formal knowledge of one or more academic disciplines. But another aspect of teacher effectiveness may be informal, or tacit knowledge of how to handle challenging situations, or even crises, that arise in the classroom. Tacit knowledge is what a person needs to know to succeed in an endeavor that is typically not explicitly taught and that often is not even verbalized. It is procedural knowledge, and thus is not just a static form of knowledge, but rather, knowledge in use. We constructed a measure of tacit knowledge for elementary-school teachers, in order to determine the teachers' likely effectiveness in dealing with problematical classroom situations. In Study 1, our primary goal was to determine whether our measure predicted principals' ratings of the teachers' classroom performance as well as teachers' ratings of their own effectiveness. We found that teachers who scored higher on our tacit-knowledge inventory generally were rated as "more" effective by their principals, but as "less" effective by themselves. In Study 2, we investigated whether responses to problematical situations that were viewed as better or worse, respectively, in the United States were also viewed as better or worse, respectively, in Israel. We found a high correlation between responses in the two countries. We concluded that it is possible to measure an important aspect of teacher effectiveness via a measure of tacit knowledge, and that comparable measurements may be possible across at least two cultures. (Contains 4 tables.)